Measuring the thickness of a ferromagnetic or paramagnetic material can be very important but extremely difficult in many applications. These types of materials, e.g., carbon steel and iron, are utilized in numerous applications. These material are selected because of their strength and durability. However, such materials are subject to corrosion or erosion over time. The most common example of corrosion is the rust that forms on such material due to oxidation. This corrosion can destroy the strength of the material. For example, a device can be associated with a logging tool and used to check oil well casings for corrosion. Another example is a device that can be mounted on or moved along the outside of a storage tank to test for corrosion inside the tank. This may be useful when access to the inside of the tank is inconvenient, difficult or costly.
In many applications, it is not possible to have access to all parts of the structural steel or iron. Examples include but are not limited to underground pipes, underground storage tanks, above ground storage tanks such as chemical tanks, and bridge and building structural elements. This is also experienced in well casings placed in the ground. The casing is subject to corrosion from the moisture in the ground on the outside of the casing as well as upon the interior surface of the casing. However, only the interior surface is accessible. The ability to monitor the changing thickness of the casing over time can provide useful information regarding the remaining strength of the casing as it experiences corrosion on the outer surface.
Methods for determining thickness in the past have included audio soundings, x-rays and gamma rays. Utilization of these alternate methods, when possible, requires physical contact with the material and frequently causes significant disruption of production activities and time consuming placement and calibration of equipment.